This invention relates to roller cutter drill bits used primarily in drilling wells for oil, gas, or geothermal energy, and more particularly to the cutting elements used on the roller cutters.
In drilling wells the cutting elements or inserts which fit within sockets in annular rows on the roller cutter body become dull or worn resulting in inefficiency and in many cases resulting in the bits having to be removed from the hole and replaced with new bits. Generally, the prior art has attempted to minimize these problems by fabricating the cutting inserts from abrasion resistant grades of tungsten carbide. The grade of tungsten carbide selected depends on the formation to be cut, and the drilling conditions encountered. Inserts having a high abrasion resistance are typically harder and will have a greater wear life but are more brittle and thus more susceptible to fracture when drilling in hard tough formations, while inserts having a tougher grade of carbide are relatively soft and will have less wear resistance. A compromise is generally made using a grade of tungsten carbide which is as hard as possible but will not fracture when hard tough formations are encountered. As is generally the case, the hard tough formations are encountered only occasionally, usually in a shallow layer or in nodules of formations. This is commonly referred to as hitting a "hard streak". Because of these "hard streaks" in the formations encountered, occasionally a tougher less abrasive resistant cutting insert is generally employed. If it were not for these occasional "hard streaks" a harder more abrasive resistant insert could be used and less insert wear would occur.
The very abrasive formations encountered usually contain a high degree of crystalline silica. If this formation is loosely bonded, such as a weak sandstone, then it is easily fractured and is not prone to cause any insert fracture problems. On the other hand, if it is a cryptocrystalline silica such as chert, and has no distinct grains it is hard and tough to fracture and would be prone to cause insert fracture problems. Fortunately, as mentioned above this type of formation occurs only occasionally and generally in shallow layers or small nodules.
During drilling, a rock bit usually encounters stratas of abrasive and non-abrasive formations, as in the typical case of sand and shale sections. In general the degree of abrasive formation encountered will control the rate at which the inserts wear and since the inserts in the same annular row on the roller cutter are of generally the same hardness the wear will occur at roughly equal rates for all of the inserts in the row. As the inserts wear, their cutting efficiency becomes diminished, causing the drilling rate to decrease which will generally control the time at which the bit is replaced with a new sharp bit.
In most drilling situations the abrasive formations are encountered sequentially with less abrasive strata, such as the typical case of a sand-shale laminate section. In these cases, the more abrasive sand sections cause accelerated wear while little wear occurs in the less abrasive shales. Wear on the cutting surface of the insert will limit the bit's drilling rate even in the shale.
Prior art drill bits have disclosed composite inserts, using multiple grades of tungsten carbide in the fabrication of inserts in an attempt to create an abrasive resistant tough insert to reduce the wear or dulling of the insert cutting surface while maintaining toughness to prevent fracture. As examples of prior art drill bits having cutting elements or inserts formed of multiple grades of tungsten carbide, reference is made to U.S. Pat. Nos. 4,705,124; 4,722,405; and 4,694,918. Such prior art discloses cutting inserts in which the cutting surface uses a harder grade of tungsten carbide and grades to a softer material, or has a diamond layer for increased hardness. Also shown are cutting inserts formed of different grades of tungsten carbide material with the leading face of the insert using a softer tougher grade of tungsten carbide and the trailing face having a harder grade of tungsten carbide.